Question regarding namespace rules

In case anybody reading this knows the answer, I was asked about possibly standardizing a metadata extension the Shibboleth Project defined many years ago as part of the Subject-ID draft I've been working on, and the impediment is that we can't really change the XML namespace at this stage, there's just too much deployment and it would be a total waste of time to move it now.

I can't find a statement that unambiguously states it's absolutely disallowed for an OASIS specification to publish an XML schema in a non-OASIS (but legitimate) namespace, but I certainly can't find a lot of evidence that we could either. Is there a clear yes/no rule on this?

-- Scott